1. Field of the Invention
This invention generally relates to magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d) and, more specifically, to an apparatus, system and method for calibrating magnetic resonance receiver coils.
2. Description of Related Art
Magnetic resonance imaging is an imaging technique used primarily in medical settings to produce high quality images of the inside of a natural organism (such as within a human body). MRI is based on the principles of nuclear magnetic resonance, i.e., a spectroscopic technique used to obtain microscopic chemical and physical information about the molecules. In particular, MRI is based on the absorption and emission of energy of the molecules in the radio frequency (xe2x80x9cRFxe2x80x9d) range of the electromagnetic spectrum. MRI involves the use of a number of coils for generating and detecting magnetic fields.
Conventionally, magnetic resonance coils are used externally to the body to generate magnetic resonance images, while magnetic resonance microcoils may be mounted at the tip of a catheter or other insertion devices used to probe the interior of the body. With magnetic resonance coils, which produce static magnetic fields, a standard design goal is that the strength of the coils"" components in the z-direction (typically the direction parallel to the axis of the main outer coils) should be as uniform as possible over the region to be scanned. Transmit coils emit an RF pulse to which molecules in the body respond. The response is detected by a receiver coil. In cases where the transmit coil is located externally to the subject to be imaged and the receiver coil is located internally (within the subject to be imaged), the signal strength of the response at the receiver coil diminishes based on its distance from the responding molecule. As such, a good signal-to-noise ratio at the receiver coil requires either that the response be strong at the molecule, or that the receiver coil be close to the responding molecule.
A strong response at the molecule requires that it possesses a strong intrinsic magnetic moment, which is in practice a severe limitation on the types of molecules which can be usefully imaged. One way of overcoming this limitation is to make the receiver coil a microcoil in the order of millimeters or less in diameter, to be carried by a catheter or other physically invasive device. Designers of such microcoils have sought to achieve high gains and uniform responses over wide fields of view. See, for example, xe2x80x9cA Device for High Gain and Uniformly Localized Magnetic Resonance Imaging,xe2x80x9d U.S. application Ser. No. 09/532,145now U.S. Pat. No. 6,487,437; xe2x80x9cA Microcoil Device for Local, Wide Field of View and Large Gain Magnetic Resonance Imaging,xe2x80x9d U.S. application Ser. No. 09/532,667now U.S. Pat. No. 6,560,475; and xe2x80x9cA Microcoil Device with a Forward Field of View for Large Gain Magnetic Resonance Imaging,xe2x80x9d U.S. application Ser. No. 09/532,037now U.S. Pat. No. 6,587,706, all of which were filed Mar. 21, 2000 and are hereby incorporated by reference.
There are many complications, however, involved with the use of small inserted microcoils. Microcoils have highly spatially varying signal reception profiles that fall off sharply with increasing distance. Microcoils have signal reception profiles that are dependent on their orientations with respect to a magnetic resonance scanner""s static magnetic field. Microcoils"" electrical properties (inductance, capacitance, etc.) in a tissue are different from coils"" electrical properties in vacuum. Upon RF excitation, precessing atomic nuclei (typically protons in hydrogen atoms of the molecules) in the tissue emit decaying RF radiation, which induce currents both in the receiver microcoil and in the tissue. The currents in the tissue introduce coupling effects between the receiver microcoil and the tissue producing resistive, inductive and capacitive consequences. The complications are not uniform, e.g., the effects differ in areas such as brain tissues, cerebrospinal fluid, blood, etc. In other words, the signal reception field of a microcoil is affected by its surrounding medium and, as a result, the microcoil cannot be calibrated simply by tests in a single known environment.
In one method for calibrating microcoils which yields a uniform intensity scale over the extent of an image, the reconstructed signal intensity in an image acquired by the microcoil is renormalized by dividing it by the theoretically expected reception field profile (as dictated by the principle of reciprocity in electromagnetism). A drawback of this method is that the orientation of the microcoil with respect to the magnetic resonance scanner""s static magnetic field must be known. Moreover, the changes in the signal receptivity due to the coil""s dielectric surroundings are ignored. Thus, although the use of microcoils in MRI devices has advanced in recent years, it is still a growing field with a need for an apparatus, system and method for calibrating magnetic resonance microcoils.
Exemplary embodiments of the invention include apparatuses, systems and methods for calibrating a movable receiver coil (the terms movable receiver coil and receiver coil are used interchangeably with microcoil in the present invention) in a dielectric environment. A method of the invention includes dividing the receiver coil into parts, determining electromagnetic interactions of the parts in the environment, and correlating the electromagnetic interactions of the parts so as to characterize the environment. The method of the invention further includes (1) estimating orientation and position information corresponding to the receiver coil, and (2) associating the correlated electromagnetic interactions data with records such as (a) responses of the receiver coil in various tissue environments to emissions caused by stimuli from external magnetic resonance coils, (b) high-resolution external receiver coil records of emissions from same environments, and (c) measures of self-interactions and mutual interactions among the parts of the receiver coil.
The parts of the receiver coil may be connected in series or parallel. The measures of self-interactions and mutual interactions among the parts may include records of currents flowing in the parts when each of the parts is exposed to a predetermined voltage or when a whole of the receiver coil is exposed to one or more predetermined magnetic pulses.
The method of the invention further comprises constructing a cost function using the associated records and a correction function in each tissue environment, and combining the associated records with the cost function to train an automated system to associate any set of interaction records with a scheme of modifications to functions constructed by the receiver coil. The modifications include spatially-varying functions that are added to a spatially-varying multiplier so as to convert constructed image intensities to attributed image activities. The automated system may be an artificial neural network, a genetic algorithm, a statistical least-squares fit computation, or any known or foreseeable predictors.